Move shaders into glsl and hlsl directories

Move `data/shaders` to `data/shaders/glsl`
Move `data/hlsl` to `data/shaders/hlsl`

Fix up shader paths in the cpp files to point to the new glsl location.

`data/shaders/hlsl/compile.py` still overwrites the glsl .spv files (for
now).

Issue: #723

data/shaders/hlsl/base/uioverlay.frag

@@ -0,0 +1,15 @@
+// Copyright 2020 Google LLC
+
+Texture2D fontTexture : register(t0);
+SamplerState fontSampler : register(s0);
+
+struct VSOutput
+{
+	[[vk::location(0)]]float2 UV : TEXCOORD0;
+	[[vk::location(1)]]float4 Color : COLOR0;
+};
+
+float4 main(VSOutput input) : SV_TARGET
+{
+	return input.Color * fontTexture.Sample(fontSampler, input.UV);
+}